Polysaccharide protein conjugate vaccines are made using polysaccharides, generally from the surface coat of bacteria, linked to protein carriers. The chemical bonding of the polysaccharide and protein carrier induces an immune response against bacteria displaying the polysaccharide contained within the vaccine on their surface, thus preventing disease. Accordingly, vaccination using polysaccharides from pathogenic bacteria is a potential strategy for boosting host immunity. The polysaccharides that cover bacteria vary greatly, even within a single species of bacteria. For example, in Streptococcus pneumoniae (a leading cause of meningitis, pneumonia, and severe invasive disease in infants and young children throughout the world) there are more than 90 different serotypes due to variation in the bacterial polysaccharide coat. Therefore, polysaccharide vaccines often consist of a panel of polysaccharides to increase protection.
Although polysaccharides are immunogenic on their own, conjugation of polysaccharides to protein carriers has been used to improve immunogenicity. The carrier protein can be either a related protein antigen from the target pathogen, boosting the specific immune response to that pathogen, or a generally immunogenic protein that serves more as an adjuvant or general immune response stimulant.
Multivalent pneumococcal polysaccharide-protein conjugate vaccines have been licensed for many years and have proved valuable in preventing pneumococcal disease in infants and have recently been recommended for adults.